1. Field of the Invention
The present invention relates to a rotating field sensor for detecting an angle that the direction of a rotating magnetic field forms with respect to a reference direction.
2. Description of the Related Art
In recent years, rotating field sensors have been widely used to detect the rotational position of an object in various applications such as detecting the rotational position of an automotive steering wheel. Rotating field sensors are used not only to detect the rotational position of an object but also to detect a linear displacement of an object. Systems using rotating field sensors are typically provided with means (for example, a magnet) for generating a rotating magnetic field whose direction rotates in conjunction with the rotation or linear movement of the object. The rotating field sensors use magnetic detection elements to detect the angle that the direction of the rotating magnetic field forms with respect to a reference direction. The rotational position or linear displacement of the object is thus detected.
There has been known a rotating field sensor that has two bridge circuits (Wheatstone bridge circuits) as shown in U.S. Pat. Nos. 6,943,544 B2, 6,633,462 B2, and U.S. Patent Application Publication No. 2009/0206827 A1. In such a rotating field sensor, each of the two bridge circuits includes four magnetoresistive elements (hereinafter referred to as MR elements) serving as magnetic detection elements. Each of the bridge circuits detects the intensity of a component of the rotating magnetic field in one direction, and outputs a signal that indicates the intensity. The output signals of the two bridge circuits differ in phase by ¼ the period of the output signals of the bridge circuits. The angle that the direction of the rotating magnetic field forms with respect to a reference direction is calculated based on the output signals of the two bridge circuits.
In a rotating field sensor that uses MR elements as the magnetic detection elements, the waveforms of the output signals of the MR elements corresponding to the resistance values ideally trace a sinusoidal curve (including a sine waveform and a cosine waveform) as the direction of the rotating magnetic field rotates. However, it is known that the waveforms of the output signals of MR elements can be distorted from a sinusoidal curve, as described in U.S. Pat. No. 6,633,462 B2. If the waveforms of the output signals of the MR elements are distorted, the angle detected by the rotating field sensor may include some error. One of the causes of the distortion of the output signal waveforms of the MR elements is the MR elements themselves.
A description will now be given of an example in which the output signal waveforms of the MR elements are distorted due to the MR elements themselves. Here, assume that the MR elements are giant magnetoresistive (GMR) elements or tunneling magnetoresistive (TMR) elements. A GMR or TMR element includes a magnetization pinned layer whose direction of magnetization is pinned, a free layer whose direction of magnetization varies according to the direction of the rotating magnetic field, and a nonmagnetic layer disposed between the magnetization pinned layer and the free layer. One example of the situations where the output signal waveform of an MR element is distorted due to the MR element itself is when the direction of magnetization of the magnetization pinned layer varies due to the influence of the rotating magnetic field, etc. This is likely to occur when the intensity of the rotating magnetic field is relatively high. Another example of the situations where the output signal waveform of an MR element is distorted due to the MR element itself is when the direction of magnetization of the free layer does not coincide with the direction of the rotating magnetic field due to the influence of such factors as the shape anisotropy and coercivity of the free layer. This is likely to occur when the intensity of the rotating magnetic field is relatively low.
U.S. Pat. No. 6,633,462 B2 discloses a magnetoresistive sensor including a main sensing element having a main reference magnetization axis, and two correction sensing elements having their respective reference magnetization axes inclined with respect to the main reference magnetization axis. The two correction sensing elements are electrically connected to the main sensing element to correct the detected angle. In this sensor, however, the design of the correction sensing elements needs to be optimized according to the design conditions such as the resistances, sizes and materials of the main sensing element and the correction sensing elements and the intensity of the rotating magnetic field. This poses a problem that the design of the sensor is not easy.
As has been described, a rotating field sensor that uses MR elements as the magnetic detection elements has the problem that the angle detected by the rotating field sensor may include some error. However, this problem can occur in any rotating field sensor that includes magnetic detection elements to detect the angle that the direction of a rotating magnetic field forms with respect to a reference direction.